Cell surface carbohydrates change during development suggesting that they are probably involved in differentiation. These developmentally-regulated changes allow some antibodies directed against carbohydrates to discriminate among tissues, both normal and malignant. To obtain cell-specific monoclonal antibodies, mice and rats have been immunized with various cell types in many laboratories. Some of the antibodies derived from spleen cells of the immunized animals have an apparent specificity for certain cells and developmental stages. Out of 490 of these antibodies sent to us from several laboratories in the past 5 years, about one-fourth are directed against carbohydrate sequences in glycoproteins and glycolipids. These antibodies are being used to study changes in cell surface carbohydrates during development in hopes of providing insights into the functions of glycoconjugates in growth and differentiation.